1. Field of the Invention
The present invention relates to a pacemaker of the type having an interference detecting circuit and an evaluating and storing arrangement for continuously evaluating and storing the heart rate.
2. Description of the Prior Art
It is known to devise pacemakers which, in an interference situation, revert to an "interference backup pacing" mode with a pacing rate equal to the programmed basic rate or equal to a predetermined rate which is somewhat higher than the programmed basic rate. Thus, in U.S. Pat. No. 4,091,818 a cardiac pacing apparatus is described having a first signal processing channel which functions in the demand mode and a second signal processing channel for detecting electromagnetic interference and causing the pacing apparatus to revert to a safe operating rate in the presence of such interference. This operating rate may be an appropriate predetermined fixed rate or a rate-limited rate which is synchronous with the detected electromagnetic interference.
Also U.S. Pat. No. 5,522,857 discloses a pacemaker with circuitry for determining when the sensed signals represent depolarization signals from the patient's heart and when such sensed signals are noise signals and cannot be used for control of the pacemaker. A safety arrangement is provided for controlling the pulse generator to generate a pacing pulse in response to a noise determination that occurs after time out of a predetermined time interval.
From European Application 0 713 714 a control system for medical devices is known in which electromagnetic interference of a biomedical signal used for the control of the device is determined using a correlator. The device is operated in the normal manner as long as the intensity or level of the electromagnetic interference component is such that it does not affect or interfere with the device operation. If the level of the electromagnetic interference exceeds a predetermined threshold the biomedical signal is blanked and a replacement signal, derived from either a previous normal input signal, a stored signal, or a synthesized signal is substituted for the biomedical signal.
U.S. Pat. No. 4,516,579 discloses an interference recognition circuit in a heart pacemaker for improved recognition of certain kinds of interference. The circuit is used for testing sensed heart action signals to recognize the signal as an interfering signal and then rendering the signal ineffective, or recognizing the signal as a true heart action signal and then feeding the signal to a control circuit for the pacemaker.
The backup pacing according to the prior art in interference situations is normally performed at fixed rate and asynchronously to the patient's intrinsic heart activity. Thus the backup pacing is not adapted to the patient's physiological heeds.
Thus, if a patient has e.g. a VVI-pacemaker programmed to a basic rate of 70 bpm and at a given time has an intrinsic heart rate of 85 bpm, the pacemaker will operate in the inhibited mode. If this patient enters into a strong electromagnetic field, as produced e.g. by certain types of anti-theft systems in departmental stores or of any other strong interference source, the pacemaker will not be able to discriminate intrinsic heart activity due to the electromagnetic interference voltages induced in the electrode, but will instead detect the electromagnetic interference during noise sample window and will change the mode of operation to asynchronous VOO-pacing at e.g. 70 bpm. These pacing pulses at the rate of 70 bpm can stimulate within the vulnerable phase of the EGM and induce fibrillation of the patient's heart. A number of cases have been reported in the literature of competitive fixed rate pacing introducing cardiac fibrillation. The same may happen to patients entering a static magnetic field which closes the reed-switch or the equivalent thereof which is inside the pacemaker, with the result that the pacemaker reverts to a fixed magnetic test rate pacing.
Different inductive loop radiators, e.g. electronic article surveillance (EAS) systems, are expected to increase very rapidly in the near future, and even if heart signal detection and discrimination circuits in implanted heart stimulators will become more sophisticated, the number of interference situations nevertheless will increase. This expected development requires a much more sophisticated technique or interference backup pacing in the future, which will decrease the risk of induced fibrillation of the patient's heart.